netdev - Re: [PATCH net-next] virtio_net: enable napi

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Date:   Fri, 14 Jun 2019 19:34:46 -0700 (PDT)
From:   David Miller <davem@...emloft.net>
To:     willemdebruijn.kernel@...il.com
Cc:     netdev@...r.kernel.org, jasowang@...hat.com, mst@...hat.com,
        willemb@...gle.com
Subject: Re: [PATCH net-next] virtio_net: enable napi_tx by default

From: Willem de Bruijn <willemdebruijn.kernel@...il.com>
Date: Thu, 13 Jun 2019 12:24:57 -0400

> From: Willem de Bruijn <willemb@...gle.com>
> 
> NAPI tx mode improves TCP behavior by enabling TCP small queues (TSQ).
> TSQ reduces queuing ("bufferbloat") and burstiness.
> 
> Previous measurements have shown significant improvement for
> TCP_STREAM style workloads. Such as those in commit 86a5df1495cc
> ("Merge branch 'virtio-net-tx-napi'").
> 
> There has been uncertainty about smaller possible regressions in
> latency due to increased reliance on tx interrupts.
> 
> The above results did not show that, nor did I observe this when
> rerunning TCP_RR on Linux 5.1 this week on a pair of guests in the
> same rack. This may be subject to other settings, notably interrupt
> coalescing.
> 
> In the unlikely case of regression, we have landed a credible runtime
> solution. Ethtool can configure it with -C tx-frames [0|1] as of
> commit 0c465be183c7 ("virtio_net: ethtool tx napi configuration").
> 
> NAPI tx mode has been the default in Google Container-Optimized OS
> (COS) for over half a year, as of release M70 in October 2018,
> without any negative reports.
> 
> Link: https://marc.info/?l=linux-netdev&m=149305618416472
> Link: https://lwn.net/Articles/507065/
> Signed-off-by: Willem de Bruijn <willemb@...gle.com>

Applied.